Chaining promises with RxJS

Question

I'm new to RxJS and FRP in general. I had the idea of converting an existing promise chain in my ExpressJS application to be an observable for practice. I am aware that this probably isn't the best example but maybe someone can help shed some light.

What I'm trying to do:

1. I have two promises - prom1 and prom2
2. I want prom1 to run before prom2
3. If prom1 sends a reject(err), I want to cancel prom2 before it starts.
4. I want the error message prom1 returns to be available to the onError method on the observer.

---

var prom1 = new Promise(function(resolve, reject) {
    if (true) {
       reject('reason');
    }
    resolve(true);
});

var prom2 = new Promise(function(resolve, reject) {
    resolve(true);
});

// What do I do here? This is what I've tried so far...
var source1 = Rx.Observable.fromPromise(prom1);
var source2 = source1.flatMap(Rx.Observable.fromPromise(prom2));

var subscription = source2.subscribe(
    function (result) { console.log('Next: ' + result); },

    // I want my error 'reason' to be made available here
    function (err) { console.log('Error: ' + err); },

    function () { console.log('Completed'); });

Answer 1

If I understood what you are trying to do - you need to create two deferred observables from functions that return promises and concat them:

var shouldFail = false;

function action1() {
    return new Promise(function (resolve, reject) {    
        console.log('start action1');
        if (shouldFail) {
            reject('reason');
        }
        resolve(true);
    });
}

function action2() {
    return new Promise(function (resolve, reject) {    
        console.log('start action2');
        resolve(true);
    });
}

var source1 = Rx.Observable.defer(action1);
var source2 = Rx.Observable.defer(action2);

var combination = Rx.Observable.concat(source1, source2);

var logObserver = Rx.Observer.create(

function (result) {
    console.log('Next: ' + result);
},

function (err) {
    console.log('Error: ' + err);
},

function () {
    console.log('Completed');
});

then for normal case:

combination.subscribe(logObserver);
// start action1
// Next: true
// start action2
// Next: true
// Completed

And case where fisrt promise fails:

shouldFail = true;
combination.subscribe(logObserver);
// start action1
// Error: reason

http://jsfiddle.net/cL37tgva/